list of lambdas in R

Question

I want to make an R function that returns a list of functions that each operate on a different part of a list. However, because of R's rules on scope, this seems impossible. Here's an example:

functiontest = function() {
    foo = list()
    for(i in 1:3) {
        fixer = function(s) { return(
            function() {
                return(s)
            }
        )}
        foo[[i]] = fixer(i)
    }
    return(foo)
}

functiontest()[[2]]() #returns 3

Even killing off the named lambda "fixer" and using an immediate function doesn't save me:

functiontest = function() {
    foo = list()
    for(i in 1:3) {
        foo[[i]]=(function(s) { return(
            function() {
                return(s)
            }
        )})(i)
    }
    return(foo)
}

functiontest()[[2]]() #returns 3

I want this to return 2. How can I refactor this code so that this will happen?

Thanks.

Answer 1

First of all, the problem you have faced has nothing to do with the scoping. Second, I don't think it is a good idea to call something idiotic just because you don't understand it.

In your example, all functions in a list return the same value because of the lazy evaluation mechanism in R. See help(force), which specifically addresses your question. In short, you need to force evaluation of the function at the time you call it, which can be done by adding force:

functiontest = function() {
    foo = list()
    for(i in 1:3) {
        fixer = function(s) { 
            force(s) ### <<<---This is the only difference from your code ###
            return(
            function() {
                return(s)
            }
        )}
        foo[[i]] = fixer(i)
    }
    return(foo)
}

or, using a more concise syntax:

functiontest <- function()  lapply(1:3, function(s) {s; function() s})

Example of use:

> L <- functiontest()

> L[[1]]()
[1] 1

> L[[2]]()
[1] 2

> L[[3]]()
[1] 3